While the 1/3 stress increase is all but a memory, this article explains its history & origin.
The one-third stress increase has a long history of use in Allowable Stress Design (ASD). It is most commonly used to account for the improbability of two transient loads concurrently acting at their maximum lifetime values– for example, will the design earthquake really occur at the same time as the design snowstorm? Changes in the load standard ASCE 7 (Minimum Design Loads for Buildings and Other Structures) restrict the use of the one-third increase, and accordingly, the AISC ASD Specification has been modified for consistency in Supplement No. 1 to the Specification for Structural Steel Buildings: Allowable Stress Design and Plastic Design, available as a free download from www.aisc.org/freedownloads.
The increase is often applied as a reciprocal 3/4 reduction factor on the load side of the equation, so for simplicity and uniformity, this reciprocal format is used. Coverage is included of provisions from ASCE 7-93,ASCE 7-95, ASCE 7-98, SEI/ASCE 7-02, the 2000 International Building Code.
(IBC), the 2003 IBC, the 1999 National Building Code (BOCA), the 1999 Standard Building Code, the 1997 Uniform Building Code (UBC), and the 2003 NFPA 5000. For each source, the load combination information is located and summarized, and the limits of the application of the one-third stress increase are discussed.
Engineering judgment is necessary to determine load combinations that potentially cause unfavorable effects, as the given load combinations are not meant to be all-inclusive. The following loads are shown in the load combinations given in this paper:
D Dead load
L Live load
Lr Roof live load
S Snow load
R Rain load
W Wind load
E Earthquake load
Other loads that might need to be considered are F (fluid load), H (earth load), P (ponding load), T (self-straining force), and Fa (nominal flood load). For simplicity, these loads are shown only in the combinations when they appear in the corresponding standard or code.
CONCLUSIONS
Code-specified ASD load combinations today provide for the proper amplification of loads, with one transient
load at its maximum lifetime value and other transient load(s) reduced to their arbitrary point-in-time value(s). This accomplishes what the one-third stress increase used to accomplish on the material-strength side of the equation. Consequently, the one-third stress increase usually is inappropriate for use with current ASD load combinations. Engineers that “double-dip,” i.e., use both the one-third stress increase on the material side and reduced ASD load combinations, are violating the code in an unconservative way. The summary provides a look at the majority of current model building codes and how the one-third stress increase is or is not appropriately used.
As the load standard upon which most current (and all future) model building codes are based, SEI/ASCE 7 best summarizes the current use of ASD load combinations and the one third stress increase. The reader is encouraged to become familiar with this document.
Found in the October 2003 issue of Modern Steel Construction.
Written by: Keith Mueller, Ph.D. is Senior Engineer with AISC’s Steel Solutions Center in Chicago. Charles J. Carter, P.E., S.E. is Chief Structural Engineer at AISC in Chicago.
Last Update: March 30, 2021
